1. Field of the Invention
The present invention relates to a technique of measurement for information of, for example, a waveform of an electromagnetic wave, and more particularly to a measurement device and a measurement method with the use of a signal processing technique by wavelet analysis for a terahertz wave.
2. Description of the Related Art
In recent years, non-destructive spectroscopic imaging with the use of a terahertz wave, and terahertz time-domain spectroscopy (THz-TDS) for investigating the properties of a substance with the use of a terahertz wave pulse are being actively studied. Japanese Patent Laid-Open No. 10-153547 discloses a method relating to image processing for imaging with a terahertz wave. The method uses wavelet analysis for information compression or peak detection. Also, “De-noising techniques for terahertz responses of biological samples,” Microelectronics Journal 32 (2001), pp. 943-953 discloses a method including converting a time waveform of a terahertz wave by wavelet transform, and setting a set of values of wavelet expansion coefficients that is smaller than a threshold to 0 (threshold processing). By performing inverse wavelet transform after manipulating wavelet expansion coefficients in this way, a time waveform in which the noise component is reduced can be acquired. In “De-noising techniques for terahertz responses of biological samples,” Microelectronics Journal 32 (2001), pp. 943-953, signal to noise (SN) ratios are compared with each other for various mother wavelets after noise components are reduced, and the mother wavelet that reduces the noise component most effectively is determined. The noise component is mainly a white noise that appears in all frequency ranges. The component of the white noise has not much correlation with the mother wavelet. Hence, the white noise appears near 0 for each expansion coefficient.
With the conventional technique, to increase the SN ratio of the measured time waveform of the terahertz wave, noise reduction is performed by wavelet transform (wavelet shrinkage). Unfortunately, this method may delete spectrum information that is needed for identifying a substance in the course of signal processing. The conventional method is enough if only necessary is acquiring image data with a good SN ratio. However, if spectroscopic data on a substance in an image should be acquired simultaneously when the image is acquired with the terahertz wave as the electromagnetic wave, it may be difficult to use a terahertz waveform after the signal processing is performed, to identify the substance (spectroscopic analysis).